Methods and devices for measuring tear film and diagnosing tear disorders

ABSTRACT

Methods and devices measure eye blinks and tear film lipid and aqueous layer thickness before and following ophthalmic formula application onto the ocular surface, especially wherein the ophthalmic formula is an artificial tear. The methods and devices are suitable for dry eye diagnosis. The methods and devices are suitable for use to evaluate ophthalmic formula effects on the tear film and to use such information to diagnose ophthalmic formula treatment of ocular disease conditions such as dry eye in the absence of contact lens wear or post-surgical eye drop treatment and diagnosis. The methods and devices are also suitable for use in the optimization of ophthalmic drug dosage forms and sustained drug release.

This application is a divisional application and claims priority to U.S.application Ser. No. 12/115,350, entitled Methods and Devices forMeasuring Tear Film and Diagnosing Tear Disorders, filed on May 5, 2008,now U.S. Pat. No. 7,963,655, and claims the benefit of U.S. provisionalpatent application No. 60/916,267, filed May 4, 2007, the entirecontents of both of which are hereby incorporated by reference in theirentirety for all purposes as if fully set forth herein.

BACKGROUND OF THE INVENTION

1. Field of the Invention

Embodiments of the invention relate to methods and devices for use inevaluating ophthalmic formula effects on the tear film and to use suchinformation to diagnose ophthalmic formula treatment of ocular diseaseconditions such as dry eye or post-surgical ophthalmic formula treatmentand diagnosis.

2. Description of the Related Art

Dry eye syndrome is a prevalent condition among both men and women forwhich there is no cure, although symptoms may be relieved with properdiagnosis and treatment. The condition affects more than 3.2 millionAmerican women middle-aged and older alone (Schaumberg D A, Sullivan DA, Buring J E, Dana M R. Prevalence of dry eye syndrome among US women.Am J Ophthalmol 2003 August; 136(2):318-26). Most dry eye patients areprescribed artificial tears to treat their dry eye conditions. Contactlens wearers, computer users, patients who live and/or work in dryenvironments, and patients with autoimmune disease are all particularlysusceptible to developing dry eye.

Individuals with moderate to severe dry eye are unsuitable for contactlens wear and must wear eyeglasses or undergo refractive surgery fortheir vision correction needs. LASIK refractive surgery induces somedegree of dry eye in virtually all patients for a period of time,sometimes six months or more. Cataract surgery also induces some degreeof dry eye in a substantial number of patients for a period of time. Itmay be desirable to prescribe artificial tears for LASIK and cataractsurgery patients to treat their dry eye condition.

Current methods for diagnosing dry eye in the absence of contact lenswear utilize methods such as symptom assessment, fluorescein staining,tear film break-up time (TBUT), non-invasive tear film break-up time(NITBUT), Schirmer test, Phenol red thread test, rose bengal orlissamine green staining, conjunctival hyperemia, tear film osmolarity,tear lactoferrin, impression cytology, brush cytology, “tearassessment”, blink frequency and maximum interblink interval. Forvarious reasons, all of these methods are imperfect and lacking inprecision.

Symptom assessment is most often used for dry eye diagnosis, in theabsence or presence of contact lens wear. It is a subjective andqualitative assessment, but was nonetheless used in 82.8% of alldiagnoses of dry eye in a recent study (Nichols K K, Nichols J J, ZadnikK. Frequency of dry eye diagnostic test procedures used in various modesof ophthalmic practice. Cornea 2000 July; 19(4):477-82).

Fluorescein staining of the cornea is frequently used for dry eyediagnosis, being used in 55.5% of all diagnoses in a recent study(IBID). This is a semi-quantitative assessment which typically consistsof dividing the cornea into 5 sections and assessing staining intensityon a 5-point scale. Most fluorescein staining methods also include anassessment of % surface area of staining within each corneal sectionafter the instillation of fluorescein dye into the eye.

Tear film break-up time (TBUT) is another test which is relativelyfrequently used for dry eye diagnosis, with or without contact lenswear. It was used in 40.7% of all diagnoses of dry eye in a recent study(MD). The tear film is a continuous film covering the eye. However, itis unstable and breaks up after a short period of time. In patients withthy eyes, the tear film breaks up faster. TBUT measurements arefacilitated with the use of fluorescein instilled into the eye with theuse of a fluorescein strip. However, the instillation of fluoresceinoften stimulates reflex tearing, obviating the measurement of TBUT.Also, the presence of fluorescein in the tear film changes theproperties of tears, which means that the measurements may not be trulyphysiological. TBUT measurements are also not precise.

The non-invasive tear film break-up time (NITBUT) method was developedto overcome the limitations of the TBUT method. With the NITBUT method,the eye is observed with a keratometer, hand-held keratoscope ortearscope. The reflections of keratometer mires are observed and thetime is measured for a mire to break up following a blink. There isnonetheless considerable variation of NITBUT measurements. Furthermore,tear breakup time is abnormal in many different dry eye states and thuscannot easily differentiate between thy eye types.

The Schirmer test measures the amount of aqueous tears that can beproduced by the eye in 5 minutes. If too little aqueous tears areproduced, this is indicative of an aqueous deficient dry eye. If enoughtears are produced, but symptoms of dry eye exist, this is indicative ofan evaporative dry eye for example due to a lipid deficiency,blepharitis or Rosacea. In the Schirmer test, a 35 mm×5 mm filter paperstrip is placed into the lower cul-de-sac of the eye and allowed to wetover its length over 5 minutes. Schirmer tests are performed without andwith prior application of an anesthetic eyedrop. When an anestheticeyedrop is not used, the test is considered to measure basal+reflex tearsecretion. When an anesthetic eyedrop is used, the test is considered tomeasure only basal tear secretion. Most clinicians regard the Schirmertest as unduly invasive and of little value for the diagnosis of mild tomoderate dry eyes. The test cannot properly diagnose lipid deficient dryeyes. The Schirmer test cannot properly diagnose dry eye states whereinsufficient or even excess aqueous tears are produced. The test, with orwithout prior use of an anesthetic eyedrop, is also considered to lackprecision and accuracy. There is considerable overlap in Schirmer testvalues between patients with Keratoconjunctivitis sicca (dry eye) andnormals.

The Phenol red thread test was developed as a less invasive method thanthe Schirmer test. It involves the use of a cotton thread impregnatedwith phenol red dye. The dye changes color from yellow to red whencontacted by aqueous tears. The crimped end of a 70 mm long thread isplaced in the conjunctival formix. After 15 seconds, the length of thecolor change in the thread is measured in millimeters. This test is alsononetheless still invasive and lacks sufficient precision and utilityfor mild to moderate dry eye diagnosis.

Rose Bengal staining is infrequently used as a dry eye diagnostic. Thetest involves the instillation of rose bengal dye into the eye and thenperforming a visual assessment of conjunctival staining. Rose Bengalstaining is dependent upon secondary changes in the ocular surfacecaused by the primary changes due to dry eye and is a good parameter foraqueous tear deficiency only in the absence of other ocular surfacediseases.

Conjunctival hyperemia is a subjective assessment of ocular redness.Since redness occurs in ocular conditions other than dry eye (e.g.,during infection), this test is unsuitable as an independent diagnosticfor dry eye.

Osmolarity, lactoferrin, impression cytology and brush cytologydiagnostic methods all involve substantial chemical laboratory work andare thus not suited for general clinical use. Osmolarity cannotindependently diagnose dry eye conditions wherein sufficient or evenexcess aqueous tears are produced.

Tear assessment includes an assessment of total aqueous tear fluidvolume via an assessment of inferior tear meniscus height, inferior tearmeniscus radius of curvature or meniscus area. Since this method doesnot evaluate the tear film lipid layer, it is not accurately diagnosticfor tear lipid deficiencies, which account for 60% or more of dry eyecases. Tear assessment also includes an evaluation of the tear filmlipid layer using a “tearscope”. Tearscope-based diagnoses exclude anassessment of the aqueous fluid volume and thus are limited.Additionally, the Keeler tearscope allows only a semi-quantitativeanalysis of the tear lipid layer, since a spectrum-color analysis of itslight source has not been conducted, allowing a correlation betweenobserved colors and thicknesses. Also, colors are still subjectivelyevaluated.

Blink frequency and maximum interblink interval (IBI_(max)) have beendetermined to correlate to dry eye status. However, both blink frequencyand maximum interblink interval measurements have not been routinelyused to diagnose dry eye due to the inherent complexity of theirmeasurement, involving video recording and video frame analysis.

Several methods have been employed for measuring the ocular retentiontimes of ophthalmic formulations such as artificial tears used to treatdry eye. Sodium fluorescein has been added to an ophthalmic formulationand the fluorescence signal has been monitored with time using a slitlamp fluorophotometer. This method suffers from at least two problems:first, the fluorescein washes out of the eye at a rate different fromthat of the formulation components of interest and secondly it diffusesinto the ocular tissue. The latter creates a source of error informulation retention time measurements as it is difficult todistinguish between fluorescence of the thin film from fluorescence fromthe tissue.

Other methods for measuring the retention times of ophthalmicformulations in the eye include gamma scintigraphy. However, thesemethods involve the use of radioisotopes and therefore necessitateexpensive equipment and a laboratory suited for the handling ofisotopes. Also, the radioactive compounds typically have low molecularweights so they too may freely diffuse out of the viscous vehicle andinto ocular tissue or be deposited on the lid margins that will resultin erroneous retention measurements.

U.S. Pat. No. 5,634,458 discloses a method for determining precornealretention time of ophthalmic formulations employing a high-molecularweight fluorescein molecule, to avoid tissue uptake of fluorescein.While this method tracks the fluorescence of the high molecular weightfluorescein to obtain a more reliable retention time, it does notmeasure tear film aqueous or aqueous+lipid layer thickness.

In the context of conducting research on the layers of the tear film,three general methods for measuring tear film layer thickness usingoptical interference have been developed, corresponding to varying oneof three parameters, wavelength of light, angle of incident light andlayer thickness, while keeping the other two parameters constant. Theseoptical interferometry methods produce varying light reflectionintensity profiles that have been called wavelength-dependent fringes,angle-dependent fringes and thickness-dependent fringes.Thickness-dependent fringes form the basis of the Keeler and Kowa DR-1instruments. Wavelength-dependent fringes arise from the illumination ofthe tear film with a measurement beam of light of varying wavelengththat intersects with a surface area of the tear film at a constantnormal or near-normal angle of incidence. Provided that the tear filmhas an index of refraction, n, intermediate between that of thesurrounding materials, e.g., air on one side and the cornea or a contactlens on the other side, and also that the refractive indices of theadjacent layers or materials are sufficiently different from oneanother, then the incident light wave will reflect from each boundarybetween layers or materials of differing refractive index. Multiplereflections will be produced, which will give rise to oscillations inthe intensity of the total reflected light as a function of wavelengthaccording to the constructive and destructive interference of themultiple reflected waves, the latter which is dependent upon therelationship between the tear film thickness, d, and the wavelength oflight, λ. Maxima (peaks or fringes) in the reflectance spectrumrepresents the wavelengths at which constructive interference occursbetween light waves reflecting at the front and back surfaces of a thinfilm and the minima (valleys) represent the wavelengths at whichdestructive interference occurs between light waves reflecting at thefront and back surfaces of a thin film.

In recent years, wavelength-dependent optical interferometers have beendeveloped for in-vivo aqueous tear film and contact lens thicknessanalysis by King-Smith et al., as disclosed in Fogt N and King-Smith P,Interferometric measurement of tear film thickness by use of spectraloscillations, J. Opt. Soc. Am. A/Vol. 15, No. 1/January 1998: 268-275;King-Smith P et al., The Thickness of the Human Precorneal Tear Film:Evidence from Reflection Spectra, IOVS, October 2000, Vol. 41, No. 11:3348-3359 and Nichols J and King-Smith P, Thickness of the Pre- andPost-Contact Lens Tear Film Measured In Vivo by Interferometry, IOVS,January 2003, Vol. 44, No. 1: 68-77.

The instruments described in the aforementioned publications are ofsimilar design and are capable of measuring the thickness of thepre-corneal or pre-lens tear film aqueous+lipid layer thickness,post-lens tear film aqueous thickness among contact lens wearers,contact lens thickness and corneal epithelial thickness. The instrumentscan also measure the thinning or thickening rates of the various tearfilm layers during normal blinking and between blinks or over time. Theinstruments have a high degree of quantitative accuracy and precision.However, it is reported in the January 2003 IOVS reference that theinterferometer in that reference, the best of the three systems in theaforementioned three references, cannot measure mean thicknesses of lessthan 1 micron, meaning it cannot measure the tear lipid layer. Tearlipid layer thickness needs to be measured separately in order todetermine aqueous-only layer thickness, as the light reflections fromthe ocular surface arise separately from the combined aqueous+lipidlayer and the lipid layer alone. Wavelength-dependent fringes cannot beobserved from the aqueous layer only. Lipid layer thickness would haveto be measured and subtracted from the combined aqueous+lipid thicknessto derive aqueous-only layer thickness. Thus, the aforementionedteaching and instruments measure thicknesses of combined aqueous+lipidlayers. Since lipid layer thickness is typically only 2% of thethickness of the aqueous layer (e.g., 60 nm vs. 3000 nm), this onlylimits the lipid layer diagnostic capability of these instruments. Thefirst interferometer described in the Fogt et al. 1998 reference uses awavelength range of 369-810 nm. The second two interferometers,described in the IOVS, October 2000 reference and the January 2003 IOVSreference, utilize a wavelength range of 562-1030 nm. The instruments inthe aforementioned three references are limited to measuring thicknessat a single spot on the eye, approximately 300 microns round, 33×350microns rectangular or 33×35 microns rectangular in the above threereferences, respectively, all at the central corneal apex. All of theaforementioned interferometers are capable of kinetic measurements oftotal tear film layer thickness, to produce thinning or thickening ratesas well as measurements of the changes in tear film thickness over time.

Despite the optical interferometer instrument capabilities disclosed inthe prior art, the effects of an ophthalmic formula topically applieddirectly onto the ocular surface on aqueous or aqueous+lipid or lipidtear film thickness have not been fully determined None of the threeaforementioned interferometry publications discloses measurements oftear film thickness over time following the application of an ophthalmicformula directly onto the ocular surface.

Optical coherence tomography (OCT) has most recently been used tomeasure changes in total tear film thickness (e.g., aqueous+lipid layerthickness) following instillation of artificial tears. 12 mm×2 mm scansof the tear film and cornea were taken at 1310±60 nm at baseline andafter instillation of 354 of artificial tears (Refresh Liquigel™,Allergan, Irvine, Calif.). Measurements were taken at 5, 20, 40 and 60minutes after instillation. The authors tested 40 eyes in 20 subjectsand found tear film thickening in all subjects, lasting about 60minutes. Direct measurements of the tear film were not possible, thustotal tear film thickness was calculated from the subtraction of thetotal tear film+cornea thickness at baseline from that afterinstillation of the artificial tears. OCT instrument repeatability forcorneal thickness was reported to be 1.5 μm. Instrument optical errorwas 3.7 μm, which was larger than the thickness of the normal tear filmitself. Thus, this method and instrument also suffers from limitedobservation capabilities. Observation and measurement of changes in theaqueous or aqueous+lipid or lipid tear film layers from baselinefollowing ophthalmic formula application are important as these allowone to measure important changes in the tear film which likely correlateto ocular surface health status, subjective comfort, optimization ofophthalmic dosage forms and drug delivery.

Given the above limitations of prior art methods for evaluation of thetear film, either alone or before and following ophthalmic formulaapplication, it would be advantageous to have new methods which do nothave some or all of the aforementioned limitations.

BRIEF DESCRIPTION OF THE FIGURES

The present invention will now be described in conjunction with thefollowing figures, in which:

FIG. 1 provides a schematic view of the utility of the presentinvention;

FIG. 2 shows a thickness measurement standard curve usingNIST-calibrated thin film thickness standards made from vapor-depositedSiO2 on silicon wafers;

FIG. 3 provides a graph showing a plot of thickness versus time for theaqueous+lipid layers of an eye treated with Systane drops;

FIG. 4 provides a graph showing a plot of thickness versus time,illustrating the reduction in thickness of the tear film followinginstillation of an ophthalmic eye drop;

FIG. 5 provides a graph showing a plot of thickness versus time,illustrating an increase in thickness of the tear film followinginstillation of an ophthalmic eye drop;

FIG. 6 provides a graph showing a plot of thickness versus time,illustrating how the tear film thinned below baseline followinginstillation of an ophthalmic eye drop;

FIG. 7 provides a graph showing a plot of thickness versus time,illustrating how the ophthalmic formula surprisingly reduced thethickness of the tear film below the baseline thickness;

FIG. 8 provides a graph showing a plot of thickness versus time,illustrating that the installation of an ophthalmic formula firstthickened the tear film and thereafter the tear film thickness returnedto baseline;

FIG. 9 shows a plot of Phenol Red thread wetting in mm, vs. thedifference between T2 and T2 (T2−T1) in minutes;

FIG. 10 shows a plot of Phenol Red thread wetting in mm, vs. T2 inminutes;

FIG. 11 shows a plot of Phenol Red thread wetting in mm, vs. thedifference between T2 and T2 (T2−T1) in minutes for two ophthalmicformulas;

FIG. 12 shows the plots of PRT wetting in mm, vs. T2−T1 in minutes fortwo ophthalmic formulas;

FIG. 13 shows a plot of Phenol Red thread wetting in mm, vs. T2 inminutes for two ophthalmic formulas;

FIGS. 14-17 shows various plots of thickness and reflectance versustime, in seconds, where the figures differ in terms of the number ofspectrum scans; and

FIG. 18 shows a Fourier-transform-frequency plot of the % reflectancevs. time plot from FIG. 16.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The present invention provides an improved device and method forevaluating a patient's tear film. With one instrument, a practitionerwill be able to accurately diagnose thy-eye, and whether it is caused bya lipid deficiency or an aqueous deficiency. Based on the results of thetear film analysis, a practitioner will also be able to select anophthalmic formulation which is specifically formulated to treat thediagnosed condition. For example, in the case of an aqueous deficiency,the practitioner could recommend an artificial tear product which isspecifically formulated to supplement the aqueous layer. Similarly, aproduct which has been formulated to supplement the lipid layer may berecommended in the case of a lipid deficiency. The practitioner willalso be able to determine if a patient is a good candidate for LASIKsurgery, and whether the LASIK patient will require an artificial tearor dry eye therapeutic post-surgery. The methods and instruments of thepresent invention may also be used by those in the ophthalmic industryto formulate and test ophthalmic products.

The present invention is directed to methods and devices for use inevaluating the tear film and to use such information to diagnoseophthalmic formula treatment of ocular disease conditions such as dryeye or post-surgical ophthalmic formula treatment and diagnosis.Embodiments of the invention relate to methods and devices for measuringeye blinks and thin layers in a mammalian eye, before and followingophthalmic formula application onto the ocular surface. The methods anddevices measure the tear film lipid and aqueous layers as well as blinkfrequency and maximum interblink interval before and followingapplication of an ophthalmic formula.

FIG. 1 provides a schematic view of the utility of the presentinvention. With one device, the practitioner will be able to determineif a patient is a candidate for refractive surgery such as FRK, lasikand cataract surgery. The practitioner will be able to diagnose dry eye,and whether it is caused by an aqueous or lipid deficiency. Thisdiagnosis may also be performed on patients subsequent to theirrefractive surgery. The practitioner will be able to recommendophthalmic formulas which are best suited for treating needs of thepatient with dry eye. The practitioner will also be able to use aninstrument according to the present invention to determine if a patientis a candidate for contact lenses and, if so, the type of ophthalmicproducts which are most suitable for that patient. The methods andinstruments of the present invention may also be used by those in theophthalmic industry to formulate and test ophthalmic products.

The present inventors have surprisingly measured a reduction inthickness in comparison to baseline of the aqueous+lipid layer in thetear film following application of an 0.1% or 0.3% hyaluronic acid (mwt600 kD-1200 kD) ophthalmic formula. Observation and measurement of areduction in the thickness of the aqueous+lipid tear film followingophthalmic formula application is important as it allows one to measurefor the first time an important change in the tear film which likelycorrelates to ocular surface health status, subjective comfort,optimization of ophthalmic dosage forms and drug delivery. Thisdiscovery is contrary to previously-held beliefs about the tear film.

Conventionally, it has been understood that topical ocular applicationof an ophthalmic formula designed to supplement the aqueous layer of thetear film will not thin that same layer. It has been believed thattopical application of an ophthalmic formula designed to supplement thetear film will only thicken and thus stabilize the tear film. The workof Creech et al. (Creech J L, Do L T, Fatt I, Radke C J. In vivotear-film thickness determination and implications for tear filmstability. Curr Eye Res 1998; 17:1058-1066) discloses that as the tearfilm break-up time increases, tear film thickness theoreticallyincreases. Thus a thinner film would be less stable, which is along-held belief. In connection with this theoretical relationship,others have shown, for example, that topical application of 0.1% or 0.3%hyaluronic acid (mwt 600 kD-1200 kD) ophthalmic formulas will increasetear film break-up time (TBUT). Therefore, it has long been assumed thatan ophthalmic formula that enhances TBUT, thickens the tear film.

As described in more detail below, the present invention providesmethods for the observation and measurement of a reduction in thicknessin comparison to baseline of the tear film following ophthalmic formulatopical application directly onto the ocular surface. This aspect of theinvention will improve the diagnosis of dry eye disease treatment andthe development of ophthalmic formula therapeutics to treat dry eye andother diseases. Methods for measuring a reduction in thickness of thetear film of an eye, in comparison to baseline, following topicalapplication of an ophthalmic formula, generally comprise the steps of:(a) employing an optical interferometer to measure tear film thickness;(b) topically applying an ophthalmic formula; (c) waiting a period oftime; and (d) employing an optical interferometer to measure tear filmthickness, wherein the optical interferometer is preferably awavelength-dependent interferometer. The accuracy of the presentinvention allows the apparatus to measure tear film thickness as onlythe aqueous layer, only the lipid layer, the combined aqueous+lipidlayer, or all three.

When the dry eye analysis is being performed, the patient is generallynot wearing any lenses to correct vision.

As described in more detail below, the present invention providesmethods for the measurement of tear production. In the diagnosis of dryeye, this aspect of the present invention may be substituted for theprior art Schirmer and Phenol red thread tests. The method of thepresent invention for the measurement of tear production generallycomprises the sequential steps of measuring baseline tear film thicknesswith an interferometer, instilling an eye drop, measuring tear filmthickness until a time T1 when tear film thickness first returns tobaseline, and measuring tear film thickness until a time T2 when tearfilm thickness returns to baseline for a second time after thinningbelow baseline. As described herein, the inventors describe this asmeasuring the rate of tear production. Depending on the depth or levelof information gathered, such measurement may be an actual measurementof production or a relative measurement. An example of a relativemeasurement would be a characterization of a subject as having low tearproduction, average tear production or high tear production.

Using the above methods, the present invention also allows for theevaluation of in-vivo ocular surface adherence and adhesion oftopically-applied molecules such as ophthalmic demulcents and polymers.

As described in more detail below, the present invention providesmethods for the measurement of blink frequency and maximum interblinkinterval. Such methods are simpler, more accurate and precise than priorart video-based methods. The methods of the present invention formeasuring blink frequency and maximum interblink interval in an eyegenerally comprise the sequential steps of projecting at least onewavelength of light from an interferometer onto the ocular surface,measuring light reflectance from the eye over a period of time, whereinsaid period of time is comprised of sequential time increments andwherein said time increments are smaller than the time wherein the upperlid intersects the light from said interferometer and wherein saidmeasuring occurs over each time increment; and analyzing lightreflectance vs. time, wherein said analyzing comprises the determinationof number of reductions of light reflectance in a time interval.

As described in more detail below, the present invention providesmethods to quantify duration of blurring of vision, especially followingophthalmic formula application. Generally, the method of measuringduration of blurring of vision following ophthalmic formula applicationcomprises the steps of measuring either or both blink frequency andmaximum inter-blink interval before ophthalmic formula application,applying said ophthalmic formula to an eye, and sequentially measuringeither or both blink frequency and maximum inter-blink interval untilsuch time that either or both blink frequency and maximum inter-blinkinterval return to their values prior to application of said ophthalmicformula.

Analysis of blink frequency and maximum inter-blink interval net blinkmeasurements according to the present invention may also be used assurrogate measures of ocular comfort. This is highly beneficial to thepractitioner, as it provides an analytical measure for something whichis typically quite subjective.

The present invention also provides the discovery that simultaneousmeasurements of tear film aqueous thickness, aqueous+lipid layerthickness, lipid layer thickness, blink frequency and maximuminter-blink interval can be made, before or after application of anophthalmic formula, to diagnose dry eye and the treatment of dry eyewith an ophthalmic formula.

Wavelength-dependent Interferometers of the present invention require aspectrophotometer with a CCD detector and a computer and computersoftware, all of which provides for fast data acquisition, storage andmanagement. Moreover, the system needs to be capable of accuratelymeasuring the very low light intensity which is reflected from the eye.Large or small spectrophotometers are suitable, although largespectrophotometers are not suitable for routine clinical practiceoutside the clinical research setting. Smaller spectrophotometers andassociated CCD detectors, such as those utilized in interferometermodels F20-NIR (950-1700 nm wavelength range, 512-element InGaAs array)and F20-EXR (400-1700 nm wavelength range, 512-element Si & InGaAsarrays) from Filmetrics, Inc. (San Diego, Calif.) are suitable. Alsosuitable are the DSR-0512 (range 280-1700 nm) and NIRX-SR (range900-2200 nm) wavelength-dependent optical interferometers fromStellarNet (Tampa, Fla.).

Other types of optical interferometers may be employed in the presentinvention. Suitable instrument types which can quantitatively measurethin film thickness, are disclosed in Optical Interferometry, Second Ed.P. Harihan ed. 2003 Elsevier Science, which is incorporated herein inits entirety by reference. The device taught herein should not beconfused with a tear scope, which is fundamentally different.

A wavefront device can be used alternatively or in addition to theinterferometer of the present invention.

One embodiment of the methods of the present invention employs awavelength-dependent optical interferometer essentiallyoptically-equivalent to those represented in the aforementionedreferences, wherein the optical eye-alignment system of the instrumentin the IOVS, October 2000 reference is used along with the remainingoptical system of the instrument in IOVS, January 2003 reference. Animproved Chromex 500 is spectrophotometer with an Andor CCD detector,Dell computer and Andor software is used in this embodiment of aninterferometer of the present invention. Tear film aqueous+lipid, lipidand aqueous layer thicknesses can be measured with this instrument.Thickness-calculation software based upon Statistica 7 from StatSoft®(Tulsa, Okla.) utilized a non-linear estimation method using theLevenburg-Marquardt algorithm applied to the polynomial:v2=−a−b*v1−c*(v1)² +d[1+(e/2d)cos((4πn _(d) cosθ*g/v1)+h)]Exp(−j/(v1)²);where v2=measured reflectance, v1=wavelength, d=R₀=(Rmax+Rmin)/2 whereR=reflectance, e/2d=amplitude=(Rmax−Rmin)/(Rmax+Rmin), n_(d)=refractiveindex of film, g=thickness of the aqueous+lipid layer, h=phase, the a, band c termns represent a 2^(nd) order polynomial used to fit the rawdata to the large slope oscillation caused by the lipid layer and theExp(−j/(v1)²) term corrects for the modulation of fringe amplitude withwavelength. Θ is the maximum angle from normal for light incidence onthe film, 9.37°. This equation can also be used to measure the thicknessof the lipid layer.

Thickness-calculation software based upon Statistica 7 utilizing theLevenburg-Marquardt algorithm applied to the following polynomial isalso used to measure the lipid-only layer:

v2=d[1+(e/2d)cos((4πn_(d) cos θ*g/v1)+h)]; where v2=measuredreflectance, v1=wavelength, d=R₀=(Rmax+Rmin)/2 where R=reflectance,e/2d=amplitude=(Rmax−Rmin)/(Rmax+Rmin), n_(d)=refractive index of film,g=thickness of the lipid layer, h=phase and again Θ is the maximum anglefrom normal for light incidence on the film. Aqueous-only layerthickness is calculated by subtracting the measured lipid-only layerthickness from the combined aqueous+lipid layer thickness.

A thickness measurement standard curve was employed usingNIST-calibrated thin film thickness standards made from vapor-depositedSiO2 on silicon wafers (FIG. 2). Combined instrument and softwareaverage error (n=7) with the thickest available standard at 1010.4 nmwas 21.0 nm. After standard curve correction, error was −1.2 nm.Absolute errors were similar for all standards. Precision error was0.13% for the 1010.4 nm standard and 0.05% for the 727.57 nm standard.The standard curve was linear, where y (meas, nm)=1.0081× (actual,m)+14.024, r2=0.9998. Given that thicker films produce more interferencefringes, error decreased with increasing thickness. Thus, aqueous-onlyand aqueous+lipid tear film thickness errors are expected to be <1%(e.g., <30 nm at 3000 nm (3.00 microns)). Thickness calculations usingthe above software are applied to the standard curve for finalcorrection.

The interferometer that was assembled by the present inventor has beenshown to accurately and precisely measure thin films to less than 50 nm.It utilizes a Chromex 500 is spectrometer with an Andor CCD detector,Dell computer, Andor operating system software and a wavelength range ofeither 460-1085 nm or 550-1085 nm, and is limited with the presentoptics to measuring a single spot on the eye, at the central cornealapex, 12.5×133 microns. This spot is produced by projecting lightthrough a 400 micron round aperture and several focusing lenses. Thissystem can be re-configured with suitable optics to measure multiplespots on the ocular surface. By way of example, and not of limitation,the source light can be passed through multiple vertically-alignedslits, all of which are then focused onto the ocular surface in the samevertical orientation. Each individual slit image is then reflected backinto the spectrometer. This system can acquire a single spectrum of thetear film in 42 milliseconds or less and can produce excellent accuracyand precision for thickness measurements with data acquisition over atime interval range of milliseconds to continuous measurements up toseveral hours or more. This allows the kinetic measurement of layerthickness over time, thus allowing the calculation of layer thinning orthickening rates. It also allows for the measurement of blinks and thedetermination of blink frequency and maximum interblink interval.

Generally, shorter wavelength ranges of 460-1085 nm, 550-1085 nm, or550-1100 nm are acceptable for measuring tear film aqueous, combinedtear film aqueous+lipid layer thicknesses and lipid layer thicknesses,based upon the relatively large thicknesses of these layers inrelationship to the wavelengths of light within these aforementionedranges and the number of interference fringes produced. 400 nm is thepreferred cutoff at the lower limit of the wavelength range since thisrepresents the lowest wavelength of visible light. In some cases,however, it may be advantageous to use a lower wavelength limit of 350nm, to achieve greater accuracy and precision in measuring very thinlipid films at <30 nm thickness. Wavelengths below 400 nm are consideredultraviolet (UV) radiation.

Embodiments of the invention are also directed to wavelength-dependentinterferometers with wavelength ranges preferable for measuring thinfilms or layers less than 200 nm in the eye. Tear film lipid layers aretypically 30-200 nm thick, whereas tear aqueous layers are typicallygreater than 1 and less than 20-30 microns thick, depending upon whetheran ophthalmic formula eye drop has been instilled into the eye toproduce, at least transiently, an increased tear film aqueous layerbeyond the normal highest non-supplemented aqueous thickness of about 5microns. The wavelength-dependent interferometers of the presentinvention can measure thin lipid and aqueous films (layers) in the eyeand can simultaneously measure thin lipid and aqueous films and combinedlipid and aqueous films and corneal epithelial thickness in the eye.Accurate and precise lipid layer measurement is achieved preferably bythe use of longer wavelength ranges, e.g., greater than 800 nm in width,within a wavelength range from 350 to 2200 nm. Preferably, an upperwavelength value of either 1700 nm or 2200 nm is used along with awavelength range greater than 800 nm. The wavelength-dependentinterferometers of the present invention can also measure layerthickness over time and thus can determine a thinner tear film layer ata particular time interval or kinetic thinning or thickening ratemeasurements. They can also measure blinks simultaneously with thicknessmeasurements. This measurement assists with the diagnosis of dry eye andthe analysis of the outcome of dry eye therapy or therapy for otherocular diseases. As may be evident, the precision of the presentinvention, which takes measurements without impacting the tear film, hasadvantages over current diagnostic tools and methods for ocular diseaseconditions such as dry eye. The methods and interferometers of thepresent invention can be utilized for dry eye diagnosis, ophthalmicformula development, optimization of ophthalmic dosage forms,optimization of drug delivery, evaluation of therapeutic treatment ofocular disease and improvement of therapeutic treatment and ocularcomfort for the subject with dry eye or the subject who has had LASIK orcataract surgery.

Single or multiple spots of light can be focused on the tear film formeasurements. Spots are produced by shining the light source through anaperture for a round spot or a slit for a rectangular spot. Spot size onthe eye is produced from about 50 to about 400 um diameter round orabout 25×50 to about 100×1000 um rectangular slits. Several such roundor rectangular spots can be employed. Where multiple spots are desired,multiple apertures or slits can be employed.

Light sources employing voltage and current-regulated power supplies andTungsten-halogen bulbs can be used to produce light of the desiredwavelength ranges and safety. Other types of bulbs can also be employed.The relevant bulb parameters are wavelength and luminosity output,filament and bulb size, power and heat dissipation requirements and bulblifetime. A variety of suitable bulbs can be found on donsbulbs.com.Examples include the EDW/6V/108W microscope bulb, which has a straightribbon filament. Bulbs with straight ribbon filaments have the advantagethat the image of the light source on the surface of the eye and thesurface of the spectrophotometer CCD array can be more focused andintense than that of a coiled filament. This is because a straightribbon filament presents a single focal plane in the direction of theoptical light path as opposed to a coiled filament ent which hassections of coil nearer and further away from a focusing lens. Anoptical system employing a ribbon filament bulb will produce photons ofgreater coherency, maximizing the signal to noise ratio of reflectedlight. Another example of a suitable bulb is the AvantesAvalight-HAL/HL-60005 tungsten halogen bulb, which produces a wavelengthrange of 360-1700 nm. Other light sources can be used, which producelight wavelengths outside of the desired range. In this case, thedesired wavelength range can be produced using light filters inconjunction with such bulbs. For example, if a light source produceswavelengths below 400 nm in the UV range, the UV radiation can beblocked with a filter such a Schott GG435 longpass glass color filter,which blocks light below 390 mu. This filter is available from EdmundOptics in Barrington, N.J. as Edmund part no. G32-752. Filters can beplaced just in front of the light source, just in front of the eye andany place in-between. Filter placement just in front of the light sourceis preferred, at a location where light enters a fiber-optic or opticalcomponent conduit (guide), so that only filtered light enters theconduit.

Embodiments of the present invention also include wavelength-dependentinterferometers with fiber-optic or short-path optical conduits (guides)to achieve a compact size suitable for routine clinical use. Fiber-opticoptical conduits have the added advantage of preventing external lightfrom adding noise to the background signal and thus can enhance themeasurement signal and instrument sensitivity. Light is transmitted tothe eye and the reflected light is returned to a spectrophotometer inthe wavelength-dependent interferometers of the present invention. Fiberoptics (e.g., non-coaxial and co-axial fibers, fibers that transmit thedesired light radiation wavelength range and fibers that block undesiredUV or long-wavelength IR radiation outside of the desired lightradiation wavelength range) and short-path optics employingnon-fiber-optic optical components separated by space are used totransmit the light radiation to the eye and return the reflectedradiation back to a spectrophotometer. Short-path is defined herein asthe maximum total optical free-space path measured between any twooptical elements of less than 37 cm.

Fiber optic light conduits (guides) such as those from Edmund Optics canbe employed in fiber optic-based systems. High transmission glass fiberbundles such as Edmund part nos. G40-639 or J38-659 (Edmund catalogNO78C) are preferred, which have broad light wavelength transmissionfrom about 400 nm to 1800 nm. Fiber optic light guides are oftenconstructed with individual fibers packed in a hexagonal close-packedarray. The outermost ring of fibers can be used to either transmit lightto the eye or transmit light from the eye to the spectrophotometer. Theremaining fibers can be used to transmit light in the alternate lightpath in the opposite direction, that is, one group of fibers can be usedto transmit light to the eye and the other group of fibers can be usedto transmit light from the eye back to the spectrophotometer. In thiscase a separation in the total fiber bundle is employed to physicallyseparate the two groups of bundles so that they can be physicallyconnected to their respective components, such as the spectrophotometeror the light source.

Glass fibers have the added advantage that they do not transmit UV orfar IR light radiation, and thus in some embodiments of the presentinvention, the glass fibers themselves can serve as the UV and/or IRwavelength filter. Other materials can be used in the fiber guides, asis well-known in the art. Fibers which transmit multiple spots of lightto the tear film and collect the reflected light from these spots canalso be employed. Standard components for coupling fiber optic guides toother parts of the interferometer can be used and are well-known in theart.

Short-path optical conduits employing non-fiber-optic optical componentsseparated by space are used in some embodiments of the present inventionto transmit the light radiation to the eye and return the reflectedradiation back to a spectrophotometer. Common optical components such aslenses, slits, beam splitters, apertures, minors, prisms and polarizerscan be used. Two and three element achromatic lenses are preferredlenses, as they correct for on-axis spherical and chromatic aberrations,thus yielding better image quality. One key optical design criterion isto avoid the use of thin films in the optical paths, whether fiber opticor other optical components are used. This eliminates the production ofbackground spectral oscillations arising from the optical system of theinterferometer. Where more than one glass beam splitters are employed,the thicknesses should ideally be different, e.g., one 2 mm thick andthe other 3 mm thick, again to avoid the possible production ofbackground spectral oscillations which may arise for example from twonearly identical 2 mm thick beam splitters. In other words, when 2 ormore reflective beam splitters are used with a thickness differencewhich falls within the thickness measurement capability of theinstrument, this may cause background spectral oscillations. This isnormally not an issue with lenses that transmit light. The point is toavoid the creation of an artificial thin film from the combination ofoptical elements within the system. Any such problem can be detected andcorrected in the construction of the instrument, however.

The period of time that one waits after topical formula applicationbefore taking measurements can be a few seconds to 24 hours. Generally,a baseline measurement of tear film layer thickness is made, followed bya series of measurements separated by periods of time ranging from a fewseconds to minutes to hours. Where the ophthalmic formula is anartificial tear formula, the series of measurements is generally spaceda few minutes apart in the first 30 minutes after topical application ofthe formula to the ocular surface, and thereafter a series ofmeasurements is taken over successively longer time periods, typicallytens of minutes to ½ hours or longer apart. This time may be reduced bycomparing data taken during a shorter time period with a standardizedgraph, chart or data which has been developed based on an analysis of alarger patient group.

While not wanting to be bound by theory, the observation and measurementof a thinner tear film following ophthalmic formula topical applicationdirectly onto the ocular surface may be a manifestation of ocularsurface adsorption and adherence of ophthalmic demulcent molecules andpolymers. Such molecules and polymers are designed to treat dry eyedisease.

The methods of the present invention can be utilized to measure theeffects on tear film thickness of a topically applied ophthahnicformula, particularly where the ophthalmic formula comprises anophthalmic demulcent. When the present invention is used in this manner,it may be a tool which may be used to assist in formulating anophthalmic formula. By way of example, the ophthalmic demulcent can beselected from the group consisting of: carboxymethylcellulose sodium,hydroxyethyl cellulose, hydroxypropyl methylcellulose, methylcellulose,Dextran 70, Gelatin, glycerin, polyethylene glycol 300, polyethyleneglycol 400, Polysorbate 80, propylene glycol, polyvinyl alcohol andPovidone.

The ophthalmic formula may also comprise a polymer which is notclassified as a USFDA ophthalmic demulcent. The polymer may be selectedfrom the group consisting of anionic, neutral and cationic viscositypolymers. The viscosity polymer may be a viscoelastic polymer such ashyaluronic acid or sodium hyaluronate. The viscosity polymer can beselected from the group comprising hyaluronic acid, hydroxypropyl guar,tamarind seed polysaccharide, and other plant-derived polymers.

Polymers such as the following polyanionic components may be included inthe ophthalmic formulas used or formulated in association with thepresent invention: anionic cellulose derivatives, anionic acrylicacid-containing polymers, anionic methacrylic acid-containing polymers,anionic amino acid-containing polymers and mixtures thereof. A suitableclass of polyanionic components are one or more polymeric materialshaving multiple anionic charges. Examples include, but are not limitedto: metal carboxy methylcelluloses, metal carboxymethylhydroxyethylcelluloses, metal carboxy methylstarchs, metal carboxymethylhydroxyethylstarchs, hydrolyzed polyacrylamides andpolyacrylonitriles, heparin, glucoaminoglycans, chondroitin sulfate,dermatan sulfate, peptides and polypeptides, alginic acid, metalalginates, homopolymers and copolymers of one or more of: acrylic andmethacrylic acids, metal acrylates and methacrylates, vinylsulfonicacid, metal vinylsulfonate amino acids, such as aspartic acid, glutamicacid and the like, metal salts of amino acids, p-styrenesulfonic acid,metal p-styrenesulfonate, 2-methacryloyloxyethylsulfonic acids, metal2-methacryloyloxethylsulfonates,3-methacryloyloxy-2-hydroxypropylsulonic acids, metal3-methacryloyloxy-2-hydroxypropylsulfonates,2-acrylamido-2-methylpropanesulfonic acids, metal2-acrylamido-2-methylpropanesulfonates, allylsulfonic acid, metalallylsulfonate and the like.

Among the polypeptides which may be included in the ophthalmic formulasof the present invention, are galectins and mucins, which include thosewhich are naturally present in the tear film of humans. Galectinsinclude those disclosed in U.S. Pat. No. 7,189,697 B2, which isincorporated herein in its entirety by reference.

One of ordinary skill in the art will be able to see that conceptssimilar to those disclosed herein may be used to measure the flap whichis cut in association with refractive surgery.

Example 1

In this example, the combined aqueous+lipid layers tear film thicknesswas measured in the right eye of a subject (Subj 20, B2W1) prior to theapplication of Systane drops. The aqueous+lipid layers combinedthickness was measured with a wavelength-dependent opticalinterferometer of the type disclosed in King-Smith, P E et al. TheThicknes of the Human Precorneal Tear Film: Evidence from ReflectionSpectra. Invest. Ophthalmol. Vis. Sci. 2000 October; 41(11): 3348-3359,which is incorporated herein by reference in its entirety. 50measurements were taken of the tear film at a 12.5×133 um spot at theapex of the cornea, each 504 msec, over a 25.2 second interval, to yielda baseline pre-eye drop combined aqueous+lipid layers tear filmthickness of 2.03±0.49 microns. Thereafter, a single 40 uL drop ofSystane drops, lot 62314F, exp 11/07 (Alcon Laboratories, Fort Worth,Tex.), was instilled into the right eye of the same subject and thecombined aqueous+lipid layers thickness was measured several times overa period of 1 hour. It can be seen in FIG. 3 that the instillation ofthe ophthalmic formula eye drop thickened the tear film over a period oftime. Although general thickening of the tear film following topicalapplication of an ophthalmic formula is conventionally expected from theprior art, it is surprising that measurement of tear film thickness at asingle spot on the ocular surface yields a smoothly decaying functionalrelationship between thickness and time. This relationship isinterpreted as providing the surprising discovery that these single spotmeasurements are adequately representative of retention of the instilledfluid volume.

Example 2

In this example, the combined aqueous+lipid layers tear film thicknesswas measured in the right eye of a subject (Subj 12, B2W1) prior to theapplication of Systane drops. The aqueous+lipid layers combinedthickness was measured using the instrument of example 1, to yield abaseline pre-eye drop combined aqueous+lipid layers tear film thicknessof 2.99±0.15 microns. Thereafter, a single 40 uL drop of Alcon® Systanedrops, lot 62314F, exp 11/07 (Fort Worth, Tex.), was instilled into theright eye of the same subject and the combined aqueous+lipid layersthickness was measured several times over a period of 1 hour, asindicated in Table 1.

TABLE 1 time, min 0 0.88 2.75 5.92 9.07 12.87 15.05 19.93 31.12 60.55thickness, microns 2.99 10.21 5.03 3.35 2.75 2.59 2.48 2.25 2.27 1.96std dev, microns 0.15 0.53 0.29 0.24 0.17 0.19 0.16 0.23 0.17 0.22rtbaseline p < 10e−6 p < 10e−6 p < 10e−6 p < 10e−6 p < 10e−6 p < 10e−6

T-test statistical comparisons were calculated between the baselinethickness and the thickness values at 9.07 minutes and thereafter. Asignificant difference was found between these values and the baselinethickness, as indicated in Table 1. The results surprisingly indicatethat the combined aqueous+lipid layers tear film thickness was thinnerthan the baseline tear film thickness a short time after theinstillation of the Systane eye drop. FIG. 4 illustrates the reductionin thickness of the tear film following instillation of Systane. It hasbeen further discovered herein, that topical ocular application ofSystane eye drops results after a short period of time (on average after28.13 minutes), a thinner tear film in 10 out of 22 subjects in anin-vivo test of the methods of the present invention.

Not wishing to be bound by any particular theory, it is believed thatthe tear film thins after topical application of Systane eye dropsbecause of the ocular surface adsorption and adhesion of a gel matrix ofpolyethylene glycol 400 and propylene glycol ophthalmic demulcents alongwith boric acid and the polymer hydroxypropyl guar. This gel matrixforms a new ocular surface interface with the tear film aqueous layerafter a short period of time, on the order of a few minutes, resultingin the establishment of a new equilibrium tear film thickness value.Thus, the methods of the present invention can be used to evaluatein-vivo the ocular surface adherence and adhesion of topically-appliedmolecules such as ophthalmic demulcents and polymers.

Example 3

In this example, the combined aqueous+lipid layers tear film thicknesswas measured in the right eye of a subject (Subj 20, B1W1) prior to theapplication of a commercially available eye drop, Blink® tearslubricating eye drops (Advanced Medical Optics, Inc., Santa Ana,Calif.). The aqueous+lipid layers combined thickness was measured usingthe instrument of example 1, to yield a baseline pre-eye drop combinedaqueous+lipid layers tear film thickness of 1.89±0.24 microns.Thereafter, a single 40 uL drop of Blink® tears, was instilled into theright eye of the same subject and the combined aqueous+lipid layersthickness was measured several times over a period of 1 hour. It can beseen in FIG. 5 that the instillation of Blink® Tears thickened the tearfilm over a period of time. Again, although general thickening of thetear film following topical application of an ophthalmic formula isconventionally expected from the prior art, it is surprising thatmeasurement of tear film thickness at a single spot on the ocularsurface yields a smoothly decaying functional relationship betweenthickness and time. The interpretation that these single spotmeasurements are adequately representative of retention of the instilledfluid volume is reinforced by the comparison of ocular retention timemeasurements of Blink® Tears (which contains hyaluronic acid), using themethods and devices herein to ocular retention time measurements ofhyaluronic acid using a prior art method. The average retention time(e.g., time of first return to baseline thickness) of Blink® Tears usingthe methods and devices herein in 22 subjects was 39.45±31.65 minutes.This compares to the previously reported value (5 half-life time) of26.75±12.41 minutes retention time of a 0.2% 4000 kD unpreserved sodiumhyaluronate artificial tear solution in buffered saline, measured withgamma scintigraphy (Snibson G R, Greaves J L, Soper N D, Tiffany J M,Wilson C G, Bron A J. Ocular surface residence times of artificial tearsolutions. Cornea. 1992 July; 11(4):288-93).

Example 4

In this example, the combined aqueous+lipid layers tear film thicknesswas measured in the right eye of a subject (Subj 12, B1W1) prior to theapplication of Blink® Tears. The aqueous+lipid layers combined thicknesswas measured using the instrument of example 1, to yield a baselinepre-eye drop combined aqueous+lipid layers tear film thickness of2.83±0.19 microns. Thereafter, a single 40 uL drop of Blink® Tears wasinstilled into the right eye of the same subject and the combinedaqueous+lipid layers thickness was measured several times over a periodof 1 hour. It can be seen in FIG. 6 that the instillation of Blink®Tears surprisingly reduced the thickness of the tear film in thissubject below the baseline thickness after 20 minutes and that the tearfilm thickness remained thinner than the baseline thickness for morethan an hour. Based on this finding, this subject was believed to have adry eye with a low tear flow rate, resulting in a slow washout of theinstilled eye drop.

Example 5

In this example, the combined aqueous+lipid layers tear film thicknesswas measured in the right eye of a subject (Subj 22, B2W1) prior to theapplication of an ophthalmic formula eye drop containing 0.20% 800 kDmwt hyaluronic acid. The aqueous+lipid layers combined thickness wasmeasured using the instrument in example 1, to yield a baseline pre-eyedrop combined aqueous+lipid layers tear film thickness of 2.93±0.27microns. Thereafter, a single 40 uL drop of the hyaluronic acidophthalmic formula, was instilled into the right eye of the same subjectand the combined aqueous+lipid layers thickness was measured severaltimes over a period of 1 hour. It can be seen in FIG. 7 that theinstillation of the 0.20% 800 kD mwt hyaluronic acid ophthalmic formulasurprisingly reduced the thickness of the tear film below the baselinethickness after 15 minutes and that the tear film thickness remainedthinner than the baseline thickness for an hour.

Example 6

In this example, the combined aqueous+lipid layers tear film thicknesswas measured in the right eye of a subject (Subj 5, B2) prior to theapplication of an ophthalmic formula eye drop containing 0.20% 800 kDmwt hyaluronic acid. The aqueous+lipid layers combined thickness wasmeasured as in example 1 using the instrument in example 1, to yield abaseline pre-eye drop combined aqueous+lipid layers tear film thicknessof 3.12±0.11 microns. Thereafter, a single 40 uL drop of the hyaluronicacid ophthalmic formula, was instilled into the right eye of the samesubject and the combined aqueous+lipid layers thickness was measuredseveral times over a period of 1 hour. It can be seen in FIG. 8 that theinstillation of the 0.20% 800 kD mwt hyaluronic acid ophthalmic formulafirst thickened the tear film and thereafter the tear film thicknessreturned to baseline. This first return to baseline is T1 and occurredat 19.97 minutes, where the tear film was 2.93±0.31 microns thick.Thereafter, the tear film continued to thin and eventually returned tobaseline again. This second return to baseline is T2 and occurred at60.8 minutes, where the tear film thickness was 3.35±0.26 microns thick.

Example 7

In this example, the combined aqueous+lipid layers tear film thicknesswas measured in the right eye of 10 subjects prior to the application ofan ophthalmic formula eye drop containing 0.20% 800 kD mwt hyaluronicacid. The aqueous+lipid layers combined thickness was measured as inexample 1 using the instrument in example 1. Aqueous tear production wasmeasured using the Phenol Red thread test (PRT). A thread, impregnatedwith the dye phenol red, was placed into the lower cul-de-sac of eachsubject's right eye and the amount of wetting of the thread inmillimeters in a given time interval was measured. Thereafter, a single40 uL drop of the hyaluronic acid ophthalmic formula (Advanced MedicalOptics, Santa Ana, Calif.), was instilled into the right eye of the samesubject and the combined aqueous+lipid layers thickness was measuredseveral times over a period of 1 hour. The instillation of the 0.20% 800kD mwt hyaluronic acid ophthalmic formula first thickened the tear filmand thereafter the tear film thickness returned to baseline. This firstreturn to baseline is T1 and occurred at various times for eachindividual subject. Thereafter, the tear film continued to thin andeventually returned to baseline again. This second return to baseline isT2 and occurred at various times for each individual subject. The T2−T1time interval is the time period of tear film thinning below baseline.FIG. 9 shows the plot of PRT wetting in mm, vs. T2−T1 in minutes. FIG.10 shows a similar plot of PRT wetting in mm vs. T2 in minutes. Thefigures show downward trends of both T2−T1 and T2 vs. PRT wetting. Thetime period of tear film thinning below baseline, T2−T1, negativelycorrelates to PRT wetting. This is because it is believed herein that anindividual with high tear production will be able to wash out theresidual instilled tear product, still remaining in the T2−T1 period,faster than an individual with low tear production. Thus, the timeperiod T2−T1 or T2 time alone, are proportional to tear production andcan be viewed as substitute measures for tear production.

Example 8

In this example, the combined aqueous+lipid layers tear film thicknesswas measured in the right eye of 6 subjects prior to the application ofan ophthalmic formula eye drop containing 0.20% 800 kD mwt hyaluronicacid (10.8 cp tear). The aqueous+lipid layers combined thickness wasmeasured as in example 1 using the instrument in example 1. Aqueous tearproduction was measured as in example 7 using the Phenol Red thread test(PRT) in the right eye. Tear film breakup time, TBUT, in seconds, wasalso measured in the same eye. Thereafter, a single 40 uL drop of thehyaluronic acid ophthalmic formula, was instilled into the right eye ofthe same subject and the combined aqueous+lipid layers thickness wasmeasured several times over a period of 1 hour. The instillation of the0.20% 800 kD mwt hyaluronic acid ophthalmic formula first thickened thetear film and thereafter the tear film thickness returned to baseline.This first return to baseline, T1, occurred at various times for eachindividual subject. Thereafter, the tear film continued to thin andeventually returned to baseline again. This second return to baseline,T2, occurred at various times for each individual subject. The combinedaqueous+lipid layers tear film thickness was measured in the right eyeof a second group of 5 subjects prior to the application of anophthalmic formula eye drop containing carboxymethylcellulose (RefreshTears®, Allergan Pharmaceuticals, Irvine, Calif.; 3 cp tear). Theaqueous+lipid layers combined thickness was measured as in example 1using the instrument in example 1. Aqueous tear production was measuredas in example 7 using the Phenol Red thread test (PRT) in the right eye.Tear film breakup time, TBUT, in seconds, was also measured in the sameeye. Thereafter, a single 40 uL drop of the carboxymethylcelluloseophthalmic formula was instilled into the right eye of the same subjectand the combined aqueous+lipid layers thickness was measured severaltimes over a period of 1 hour. The instillation of thecarboxymethylcellulose ophthalmic formula first thickened the tear filmand thereafter the tear film thickness returned to baseline. This firstreturn to baseline, T1, occurred at various times for each individualsubject. Thereafter, the tear film continued to thin and eventuallyreturned to baseline again. This second return to baseline, T2, occurredat various times for each individual subject.

FIG. 11 shows the plots of PRT wetting in mm, vs. T2−T1 in minutes forboth tear formulas. FIG. 12 shows similar plots of PRT wetting in mm vs.T2 in minutes for both tear formulas.

Two subjects, one from each of the tear product groups, are outliersfrom their respective groups. These subjects had in one case a very lowTBUT (6.4 sec) and a very high TBUT (17.7 sec) in the other case. TBUTvalues averaged 10.9 seconds for both groups. A high TBUT implies a lowblink frequency, whereas a low TBUT implies a high blink frequency. Thehigher the blink frequency, the more rapidly an instilled drop will bewashed out of the eye. Thus, the individual with a low TBUT value isexpected to have a lower T2−T1 period and lower T2 time than others inthe same group. Conversely, the individual with a high TBUT value isexpected to have a higher T2−T1 period and higher T2 time than others inthe same group. With the exceptions of the two subjects with very low orhigh TBUT values, the figures show downward trends of both T2−T1 and T2vs. PRT wetting. The time period of tear film thinning below baseline,T2−T1, negatively correlates to PRT wetting. In this example, theresidual instilled lower viscosity 3 cP tear product, still remaining inthe T2−T1 period, is washed out faster than the higher viscosity 10.8 cPtear product. Thus, the time period T2−T1 or T2 time alone, areproportional to tear production and can be viewed as substitute measuresfor tear production.

Measuring T2−T1 or T2 alone to derive a proportional measure of aqueoustear production can be quicker if one uses a 1 cP saline solutioninstead of a higher viscosity tear formula. One drop of sterile, unitdose, isotonic 0.9% sodium chloride was instilled into the right eyes offour subjects in a separate test, after their baseline tear filmthicknesses had been measured. Measured T2/T1 values were 29.92/4.46,15.25/6.03, 17.07/8.95 and 12.40/7.75 minutes. Thus, it takes about 15minutes to measure tear production in this manner. A variety of eyedrops of differing composition can be used to measure tear production,including saline, buffered saline, and any commercially available eyedrop. Saline is preferred.

In summary, the method of the present invention for the measurement ofaqueous tear production comprises the sequential steps of measuringbaseline tear film thickness with an interferometer, instilling an eyedrop, measuring tear film thickness until a time T1 when tear filmthickness first returns to baseline, and measuring tear film thicknessuntil a time T2 when tear film thickness returns to baseline for asecond time after thinning below baseline. An alternative method of thepresent invention for the measurement of aqueous tear productioncomprises the sequential steps of measuring baseline tear film thicknesswith an interferometer, instilling an eye drop, measuring tear filmthickness until a time T1 when tear film thickness first returns tobaseline, measuring tear film thickness until a time T2 when tear filmthickness returns to baseline for a second time after thinning belowbaseline, and subtracting T1 from T2.

Example 9

This example illustrates interferometer scan time tests for blinkfrequency and maximum inter-blink interval determination usingwavelength-dependent interferometry with a single light spot at the apexof the cornea. The interferometer instrument of example 1 was used inthis example. Blink frequency is the number of blinks in a given timeperiod. Maximum inter-blink interval is the maximum time in secondsbetween 2 successive blinks. The upper lid during blinking will transitand block the interferometer light spot at the apex of the cornea for afinite, small period of time. During this time period, the light will beabsorbed by the outside skin of the upper lid and hence will not returnto the spectrometer. The amount of light reflection during this timeperiod will drop substantially or to zero or near zero. Thus, byanalyzing or plotting light reflectance (amount of light) at a constantwavelength vs. time, one can determine when a blink occurs and thereforeboth blink frequency and maximum inter-blink interval. Plots of lightreflectance vs. time will have downwards-directed spikes or “peaks”,each of which corresponds to a single blink. A wavelength at the centerof the spectrum range can be selected. It has been determined thatwavelength is not critical for blink frequency and maximum inter-blinkinterval determination. Thus any wavelength that an interferometeremploys can be utilized. A narrow band of several wavelengths can alsobe used, provided optical interference effects do not affect the summedlight reflectance. This method for measuring blink frequency and maximuminterblink interval is only successful when the interferometer scantime, or final spectrum data acquisition time interval, is short enoughto capture the rapid movement of the upper lid as it transits the lightbeam. In principle, one cannot predict what scan time will work for eachperson, since one would have to know the upper lid velocity profileacross the cornea as well as the palpebral aperture, the distancebetween the open lids. However, when the scan time is short enough, allsubjects can be measured. This scan time can be determined byexperiment. An additional unknown factor is that the interferometersignal to noise ratio changes with scan time. In particular, as the scantime decreases, the signal to noise ratio decreases, thus one cannotpredict if a particular scan time provides a good ratio for anacceptable spectrum. This also can be determined by experiment. Anotherunknown factor is the amount of light projected onto the eye, which alsoaffects the signal to noise ratio. If the light output from the lightbulb is too low and/or the spot size of the light projected onto theocular surface too small, not enough light will be reflected from thetear film back into the spectrometer. These factors can be determined byexperiment. Table 2 summarizes Chromex 500 is spectrometer parameters ofthe instrument utilized in example 1 for blink frequency determination.Each spectrum is acquired during a 21 millisecond exposure time. Another21 milliseconds are required for data accumulation by the computer, fora total of 42 milliseconds. Typically, 2 or more spectra are acquiredand added together, to increase the signal to noise ratio. Thus, thetotal scan time is 84 milliseconds for 2 added spectrum scans and 504milliseconds for 12 added spectrum scans. A typical data acquisitioninterval for one subject should be long enough to capture multipleblinks, or from about a few seconds to a few minutes. Preferably, it isabout 20-40 seconds. In this example, 25.2 seconds was employed fortests shown in FIGS. 13-16. 21.42 milliseconds was employed for the testshown in FIG. 16. In 25.2 seconds, 50 final spectra were acquired usinga 504 millisecond scan time and 300 final spectra can be acquired usingan 84 millisecond scan time.

TABLE 2 Chromex 500is Spectrometer parameters for blink frequencydetermination. Corresponding FIGS. FIG. 16 FIG. 15 FIG. 14 FIG. 13 FIG.12 none Exposure time (sec)/single spectrum scan 0.021 0.021 0.021 0.0210.021 0.021 Accumulate cycle time (sec) (total time for 1spectrum scan)0.042 0.042 0.042 0.042 0.042 0.042 Number of accumulations (# spectrumscans that are added) 2 4 6 8 10 12 Kinetic cycle time (sec) (# addedscans × total time/1 scan) 0.084 0.168 0.252 0.336 0.42 0.504 Number inKinetic Series (number of individual final spectra) 300 150 100 75 60 50total run time (sec) 25.2 25.2 25.2 25.2 25.2 25.2 Can correctly captureblinks yes yes no no no no

FIGS. 13 through 17 show plots of light reflectance vs. time and alsotear film thickness vs. time, measured at the same time on the same eye,using the Chromex 500 is spectrometer parameters in Table 2. No figureof reflectance vs. time using a 504 millisecond scan time and associatedinstrument parameters is shown. It is a useful method of the presentinvention to analyze or plot tear film thickness vs. time along withreflectance vs. time, as the thickness data can provide additionalconfirmation of blink occurrence, since tear film thickness increasesimmediately following a blink. FIG. 13 shows that acquiring 60 finalspectrum scans in 25.2 seconds (e.g., a 420 millisecond final spectrumscan time) is too slow to accurately determine blinking. Measured lightreflectance does not decrease enough and there is a poor match betweendecreased reflectance and increased thickness. The same is true for thetests shown in FIGS. 14 and 15, although one can see successiveimprovements over the test in FIG. 11. The test shown in FIG. 16, where150 final spectrum scans were acquired in 25.2 seconds (e.g., a 168millisecond final spectrum scan time) successfully captures all blinks,as does the test shown in FIG. 17, where final spectrum scans wereacquired at a rate of 300 in 25.2 milliseconds (e.g., a 84 millisecondfinal spectrum scan time).

The one data point at time zero on each of the graphs herein showingthickness vs. time represents the baseline tear film thicknesses priorto adding the drops.

FIG. 17 also illustrates the maximum inter-blink interval, whichoccurred between two successive blinks occurring at 14.784 and 18.144seconds, giving a value of 3.360 seconds. The Chromex 500 isspectrometer used acquires data with millisecond accuracy, resulting inmillisecond accuracy for both blink frequency and maximum inter-blinkinterval determination. It is often useful to measure both blinkfrequency and maximum inter-blink interval, as they are both known toindependently correlate to dry eye status and they have inversecorrelations with each other and dry eye status. For example, a highblink frequency, which correlates to dry eye, is accompanied by a shortmaximum inter-blink interval. Conversely, a long maximum inter-blinkinterval is accompanied by a low blink frequency among normal subjectswithout dry eye. The analysis of reflectance vs. time to determine blinkfrequency or maximum interblink interval can involve plotting and manualpeak counting for blink frequency and manual determination of timedifferences for maximum interblink interval or the employment ofpeak-picking algorithms or computer software using peak-pickingalgorithms.

Another useful method of the present invention involvesFourier-transformation (FT) of blink frequency data, to determine theblink frequency spectrum. This is illustrated in FIG. 18, which is aFourier-transform-frequency plot of the % reflectance vs. time plot fromFIG. 17. Since the time increment=0.084 sec, Fourierfrequency=blinks/0.084 sec. The maximum Fourier frequency is0.0551=frequency/0.084 sec=0.656 blinks/sec=1.52 sec/blink. Given thetotal time interval of 21.42 sec and 1.52 sec/blink, 14.05 blinksoccurred. Also, since there are typically several Fourier frequencies,one can apply a proportional weight to each frequency and then sum thevalues to determine a single weighted frequency term representing blinkfrequency.

In summary, the method of the present invention for measuring blinkfrequency in an eye comprises the sequential steps of projecting atleast one wavelength of light from an interferometer onto the ocularsurface, measuring light reflectance from the eye over a period of time,wherein said period of time is comprised of sequential time incrementsand wherein said time increments are smaller than the time wherein theupper lid intersects the light from said interferometer and wherein saidmeasuring occurs over each time increment; and analyzing lightreflectance vs. time, wherein said analyzing comprises the determinationof number of reductions of light reflectance in a time interval.

The method of the present invention for measuring maximum inter-blinkinterval in an eye comprises the sequential steps of projecting at leastone wavelength of light from an interferometer onto the ocular surface,measuring light reflectance from the eye over a period of time, whereinsaid period of time is comprised of sequential time increments andwherein said time increments are smaller than the time wherein the upperlid intersects the light from said interferometer and wherein saidmeasuring occurs over each time increment; and analyzing lightreflectance vs. time, wherein said analyzing comprises the determinationof the maximum time interval between reductions of light reflectance.

Given that the methods of the present invention can accurately measureblink frequency and maximum inter-blink interval, it is also possibleusing these methods to quantify duration of blurring of vision,especially following ophthalmic formula application. This is based uponthe known relationship between blurring and blinking. Thus, the methodof measuring duration of blurring of vision following ophthalmic formulaapplication comprises the steps of measuring either or both blinkfrequency and maximum inter-blink interval before ophthalmic formulaapplication, applying said ophthalmic formula to an eye, andsequentially measuring either or both blink frequency and maximuminter-blink interval until such time that either or both blink frequencyand maximum inter-blink interval return to their values prior toapplication of said ophthalmic formula.

Blink frequency and maximum inter-blink interval can also be used assurrogate measures of ocular comfort. The following example illustratesthis.

Example 10

In this example, combined aqueous+lipid layer thickness, blink frequencyand maximum interblink interval were measured at baseline in the righteye of a subject with dry eyes with poor ocular comfort, prior to theinstillation of an artificial tear solution. The methods of example 9were used, wherein 150 final spectral scans were acquired in 25.2seconds. Thereafter, a single 40 μl, drop of an artificial tear solutionwas instilled into the subject's right eye and comfort and combinedaqueous+lipid layer thickness, blink frequency and maximum interblinkinterval were assessed and measured after approximately 75 minutes.Table 3 presents the results. One can see the directional correlationbetween subjective comfort and the measurements of blink frequency andmaximum interblink interval. Combined aqueous+lipid layer tear filmthickness showed no such directional correlation, although a thicknessof 2.34 microns is considerably thinner than the reported thickness of2.94 microns among normals in one study (King-Smith P, Fink B, Fogt N,Nichols K, Hill R, Wilson G. The Thickness of the Human Precorneal TearFilm: Evidence from Reflection Spectra. IOVS, October 2000, Vol. 41, No.11:3348-3359) and 3.98±1.06 microns in another (Nichols J, Mitchell G,King-Smith P. Thinning rate of the Precorneal and Prelens Tear FilmsIOVS, July 2005, Vol. 46, No. 7: 2353-2361). Both studies involvedrelatively young subjects with an average age of 32 years, however. Thenumber of blinks in the measurement time interval, 25.2 seconds, matcheswell with the figures of 6.0 blinks/25.2 seconds for normals and 14.2blinks/25.2 seconds for dry eye subjects reported by Tsubota et al.(IBID). This example illustrates the need on occasion to measure blinkparameters in addition to thickness, to make a good diagnosis of dryeye.

TABLE 3 Parameter baseline 75 min Time, min 0.00 74.75 Ave. thickness,microns 2.34 2.47 s.d., thickness, microns 0.30 0.43 Comfort (1-5 scale,5 best) 1.00 5.00 Blinks in 25.2 sec 16 9.5 Max. Interblink Interval,sec 1.68 2.856

Example 11

In this example, combined aqueous+lipid layer thickness, lipid layerthickness, comfort and TBUT were measured, as indicated in Table 4, atbaseline in the right eye of subjects with dry eyes with poor ocularcomfort and subjects with good ocular comfort and no dry eye. Thethickness methods of example 1 were used, wherein 50 final spectralscans were acquired in 25.2 seconds, with the exception of subject 4,wherein 150 scans were acquired. Tear film layer thicknesses representaverages of 50 measurements, again with the exception of subject 4,where thicknesses represent averages of 150 scans. Comfort and TBUT werenot determined (n.d.) for one and two subjects, respectively. Comfortwas rated for all but Subject 4 on a scale of 1-10, with 10 being bestand on a scale of 1-5, with 5 being best for Subject 4. Subject H had avery thick aqueous+lipid layer and intermediate lipid layer, whichillustrate quantitatively for the first time the tear film thicknesseswhich heretofore have been only qualitatively known to occur inblepharitis, a condition causing dry eye. Thus, the methods of thepresent invention to measure tear film aqueous+lipid and lipid layers,to diagnose a condition of dry eye are illustrated. Subject 17 had athin aqueous+lipid layer, somewhat thin lipid layer, excellent comfort,TBUT>10 seconds and no dry eye. This is an example illustrating the needfor additional measurements of blink frequency and/or maximum interblinkinterval or tear production to make a correct diagnosis of dry eye ornormal eye status. Subject 18 had a very thick aqueous+lipid layer andthick lipid layer, excellent comfort and marginal TBUT. Here, a correctdiagnosis of normal eye condition can be made on the basis of thicknessmeasurements alone. Subject 4, reviewed in Example 10, had a thinaqueous+lipid layer and normal lipid thickness at baseline. These twomeasurements alone are insufficient to correctly diagnose the dry eyecondition. However, when thickness measurements are combined with blinkfrequency and maximum interblink interval measurements in example 10, acorrect diagnosis of ocular discomfort can be made. Lipid layerthickness for subject 4 at 75 minutes was 111 nm, which corresponds tothe high comfort (5), low blink frequency and longer maximum interblinkinterval measured at this time (Table 3). Subject 13 had a moderatelythin aqueous+lipid layer, somewhat thin lipid layer, good comfort andlow TBUT. This subject occasionally experiences dry eye. Here, thethickness measurements alone are not quite sufficient to correctlydiagnose the dry eye status with respect to subjective comfort, but docorrespond to TBUT, an accepted measure of dry eye. Blink measurementswould be helpful to diagnose comfort status in subject 13. Subject 8 hada normal aqueous+lipid layer, extremely thick lipid layer, poor comfortand low TBUT. The normal aqueous+lipid layer and extremely thick lipidlayer are consistent with a diagnosis of meibomianitis, which leads toexcess lipid production and subsequent dry eye. Here again, thethickness measurements alone are sufficient to correctly diagnose thedry eye status.

TABLE 4 Aqueous Lipid TBUT, Subject & Lipid (um) (nm) Comfort sec Subj H3.51 70 n.d. n.d. (blepharitis) Subj 17, B1 (thin) 1.31 45 9 13.1  Subj18, B2 (thick) 4.03 169 9 9.3 Subj 4, baseline 2.34 117 1 n.d. Subj 4,75 min 2.47 111 5 n.d. Subj 13, B2 (thin) 1.98 56 8 6.6 Subj 8, B2(thin) 2.69 181 4 5.7

Ideally, multiple measurements of the tear film and eye can be madesimultaneously, to obtain a more accurate and specific diagnosis of dryeye. For example, tear film aqueous+lipid layer thickness, lipid layerthickness, aqueous layer thickness, blink frequency and maximuminterblink interval can be measured simultaneously prior to or after theinstillation of an ophthalmic formula. Measurements of T2 and T1 can bemade after instillation of an ophthalmic formula to obtain an assessmentof aqueous tear production. Any of these measurements can be made incombination with one another.

1. A system for measuring a rate of tear production, comprising: anoptical instrument configured to focus light on an eye; a detectoroperatively coupled to the optical instrument and configured to capturelight reflected from the light focused on the eye by the opticalinstrument; a computer program product operatively coupled to thedetector, the computer program product having a computer-usable mediumhaving a sequence of instructions, which, when executed by a processor,causes said processor to execute a process for measuring the rate oftear production of the eye, said sequence of instructions including:monitoring a tear thickness with a measurement method; gathering datacomprising a time when the tear thickness returns to a baseline value;and calculating the rate of tear production.
 2. The system of claim 1,wherein the optical instrument is an interferometer.
 3. The system ofclaim 1, wherein the optical instrument is a wavefront device.
 4. Thesystem of claim 1, wherein the sequence of instructions further includedetermining a second time when the tear thickness returns to a baselinevalue.
 5. The system of claim 4, wherein the calculating is based on thedifference between the first time and the second time.
 6. The system ofclaim 1, wherein the sequence of instructions method further includes acomparison of the data with a standard which has been developed based onan analysis of a larger patient group.